Bull Volcanol (2003) 65:562–577
DOI 10.1007/s00445-003-0285-y
RESEARCH ARTICLE
Heidi Soosalu · Pll Einarsson · Steinunn Jakobsdttir
Volcanic tremor related to the 1991 eruption
of the Hekla volcano, Iceland
Received: 14 February 2002 / Accepted: 12 February 2003 / Published online: 4 April 2003
Springer-Verlag 2003
Abstract Volcanic tremor at the Hekla volcano is
directly related to eruptive activity. It starts simultaneously with the eruptions and dies down at the end of
them. No tremor at Hekla has been observed during noneruptive times. The 1991 Hekla eruption began on 17
January, after a short warning time. Local seismograph
stations recorded small premonitory earthquakes from
16:30 GMT on. At 17:02 GMT, low-frequency volcanic
tremor became visible on the seismograph records,
marking the onset of the eruption. The initial plinian
phase of the eruption was short-lived. During the first day
several fissures were active but, by the second day, the
activity was already limited to a segment of one principal
fissure. The eruption lasted almost 53 days. At the end of
it, during the early hours of 11 March, volcanic tremor
disappeared under the detection threshold and was
followed by a swarm of small earthquakes. At the start
of the eruption, the tremor amplitude rose rapidly and
reached a maximum in only 10 min. The tremor was most
vigorous during the first hour and started to decline
sharply during the next hour, and later on more gently.
During the eruption as a whole, the tremor had a
continuous declining trend, with occasional increases
Editorial responsibility: J.F. Lnat
H. Soosalu ())
Institute of Seismology,
University of Helsinki,
Teollisuuskatu 23, P.O. Box 26, 00014 Helsinki, Finland
e-mail: [email protected]
Fax: +354-562-9767
P. Einarsson
Science Institute,
University of Iceland,
Hofsvallagata 53, 107 Reykjavk, Iceland
S. Jakobsdttir
Icelandic Meteorological Office,
Bfflsta6́avegur 9, 150 Reykjavk, Iceland
Present address:
H. Soosalu, Nordic Volcanological Institute,
Grenssvegur 50, 108 Reykjavk, Iceland
lasting up to about 2 days. Spectral analysis of the tremor
during the first 7 h of the eruption shows that it settled
quickly, within a couple of minutes, to its characteristic
frequency band, 0.5–1.5 Hz. The spectrum had typically
one dominant peak at 0.7–0.9 Hz, and a few subdominant
peaks. Hekla tremor likely has a shallow source. Particle
motion plots suggest that it contains a significant
component of surface waves. The tremor started first
when the connection of the magma conduit with the
atmosphere was reached, suggesting that degassing may
contribute to its generation.
Keywords Hekla eruption 1991 · Volcanic tremor ·
Earthquakes · Eruption monitoring
Introduction
Hekla is one of the most active volcanoes of Iceland. It
has erupted at least 18 times since Iceland was inhabited
approximately 1,100 years ago (Gu6́mundsson et al.
1992). Hekla is located in south Iceland at the junction of
the south Iceland seismic zone and eastern volcanic zone
(Fig. 1). It is an ENE-WSW-trending ridge built up to the
height of 1,500 m above sea level by repeated eruptions
on a narrow fissure zone. The products are mainly basaltic
and andesitic lavas and tephra (rarinsson 1967).
The eruption history of Hekla since the settlement time
has in several ways been quite regular—in regard to
durations of events, repose periods separating them, and
amount and composition of volcanic products. Large
eruptions have typically occurred twice per century, the
average repose period being 55 years (rarinsson 1967;
Gu6́mundsson et al. 1992). A new pattern has emerged in
the last half-century, with smaller and more frequent
eruptions in 1970, 1980–1981, 1991, and 2000. Only very
short seismic precursors have been noticed before these
eruptions. The seismicity related to them has started very
gradually and has reached the detection threshold 25–
80 min before their onset (Einarsson and Bjrnsson 1976;
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Fig. 1 Index map of the Hekla region. Closed triangles are the
digital seismograph stations of the South Iceland Lowland (SIL)
network, and shaded triangles the analog stations. The stations
which are used in this study are named in the map. The closed
square is the strain station BUR of Linde et al. (1993). The central
volcanoes are outlined, and their fissure swarms shaded (Einarsson
and Saemundsson 1987) and the calderas hatched (Jhannesson et
al. 1990). Thick solid lines are the faults of the South Iceland
seismic zone. Thick solid lines inside the Hekla central volcano
show the 1991 eruptive sites. Dashed lines mark the glaciers, thin
lines lakes and rivers. The small index map shows the locations of
the western (WVZ), eastern (EVZ) and northern (NVZ) volcanic
zones, the South Iceland seismic zone (SISZ), and the Tjrnes
fracture zone (TFZ). All the figures are drawn using the Generic
Mapping Tools program (Wessel and Smith 1998)
Grnvold et al. 1983; Gu6́mundsson et al. 1992;
Einarsson 2000; Stefnsson et al. 2000).
In spite of the general lack of earthquakes, Hekla
eruptions appear to be accompanied by persistent volcanic
tremor. The tremor is easily observed and provides an
important parameter for monitoring purposes. The 1991
eruption is the first one for which there exist digital
seismic data, suitable for a meaningful quantitative study
of tremor. In this paper we attempt to describe the 1991
tremor, its characteristics, time history, and relation to the
eruption, using three digital and two analog seismic
stations. Sound knowledge of this is a prerequisite for a
successful alert system based on seismic observations.
Previous studies on earlier Hekla eruptions
The Hekla eruption in 1970 started on 5 May at about
21:23 GMT, when fissures opened on the SSW and W
flanks of Hekla (rarinsson and Sigvaldason 1972). At
that time there were only three permanent seismic stations
in Iceland, at 80-, 110- and 200-km distances from Hekla.
The stations started to record continuous tremor at 20:58,
about 25 min before the onset of the eruption. The tremor
amplitude increased steadily until 21:18 when there was a
sudden burst of tremors. Subsequently, numerous bursts
occurred superimposed on the continuous tremor (Einarsson and Bjrnsson 1976). In retrospect, it seems likely
that the tremor was made by superposition of numerous
small earthquakes.
The Hekla eruption in 1980 began on 17 August.
Earthquake activity related to it started to show up at
13:04. Continuous movement resembling a series of small
earthquakes was recorded at about 13:10. Definite
volcanic tremor was recorded from 13:27 on, about the
same time as the beginning of the eruption. It reached
maximum amplitude during the next hour but started to
decrease again at about 16:00. The amplitude followed
the tephra production rate. In general, during the eruption
the amplitude of the tremor appeared to follow the vigour
of the volcanic activity (Grnvold et al. 1983; Brandsdttir and Einarsson 1992).
The 1980 eruption lasted until 20 August but activity
at Hekla resumed in April 1981. Volcanic tremor similar
to the August tremor was noticeable on a seismometer
shortly after 22:00 on 8 April. The tremor amplitude
increased rapidly between 02:00 and 05:00 in the morning
of 9 April when the first tephra fall was noted. However,
the weather conditions prevented direct observations of
the eruption site. The tremor started to decrease slowly in
the morning of 10 April, and finally disappeared below
the detection level on 16 April with the last signs of the
eruption (Grnvold et al. 1983).
The seismograph stations observing
the volcanic tremor of the 1991 Hekla eruption
The Nordic seismological South Iceland Lowland (SIL)
project was started in 1988 for purposes of earthquake
prediction research in the south Iceland seismic zone
(Stefnsson et al. 1993). The SIL network of eight, digital,
three-component seismograph stations located in the
South Iceland Lowland (Fig. 1), west of Hekla, began
to collect data in the year 1990. The instrumentation
consisted of Lennartz LE-3D/1 s seismometers and
Nanometrics RD3 digitizers. The Hekla eruption was
the first significant geophysical event occurring during
the operation of this network.
The eruption started on 17 January 1991, after a short
warning time. The SIL stations recorded premonitory
earthquakes (Stefnsson et al. 1993; Soosalu and Einarsson 2002), and subsequently volcanic tremor associated
with the beginning of the eruption. The initial plinian
phase of the eruption was short-lived. Several fissures
were active during the first day. By the second day, the
activity was already limited to a segment of one principal
fissure where the main crater subsequently formed. This
eruption lasted almost 53 days, until 11 March (Gu6́mundsson et al. 1992).
During the first hours of the eruption, the station SAU
at 35-km distance (see Fig. 1) provides the main source
for tremor data. Continuous datasets recorded by SAU,
and by the station ASM 50 km from Hekla, until about
midnight of 17 January, are available. Unfortunately the
closest station at the onset of the eruption, SAU, is
already quite a distance from Hekla. However, we can see
many characteristic features of the tremor in its records.
HAU, the digital station nearest to Hekla, 15 km away,
564
was down when the eruption broke out, as a new software
version was being installed there. Only a 47-s data sample
was saved at HAU at 20:27 GMT on 17 January. During
the eruption as a whole, the HAU data are the main
material for tremor analysis.
Analog vertical component seismograph stations also
operated in the region during the eruption (Fig. 1), and
they provide material for more qualitative analysis of the
volcanic tremor. The analog station LJ has continuous
tremor records for the first days of the eruption, until its
amplitude faded under the detection threshold. The station
HE on the flank of Hekla itself was out of order during the
first part of the eruption. From 24 February on, it gives
account of the behaviour of the tremor during the last
days of the eruption.
Time history of the eruption in the light
of tremor observations
Hekla is known to be typically aseismic during volcanically quiet times (Einarsson 1991). In accordance with
this, no earthquakes were observed at Hekla before 17
January, and thus there are no signs of precursory
seismicity. Numerous small earthquakes were related to
the onset of the eruption (Soosalu and Einarsson 2002).
Sixty earthquakes were observed by the SIL system
before the onset of the eruption (16:30–17:01 GMT), and
320 events between 17:02 and midnight (see below). The
low-frequency volcanic tremor appears on the seismograms at 17:02 GMT (see below), showing that the
eruption had started.
According to Gu6́mundsson et al. (1992), the eruption
started at about 17:00. They mention that a rising eruption
column was first seen from a nearby farm at 17:05. Linde
et al. (1993) state that the surface breakout of the eruption
is not determined precisely by visual or other direct
observation. Their strain measurements show clear strain
changes due to magma movement from 16:36 on. They
inferred the surface breakout to be at 17:01 (Linde et al.
1993; Stefnsson et al. 1993), consistent with our
conclusion.
The eruption was by far most vigorous during the first
day, and several fissures were active. According to
eyewitness accounts, the eruption attained its peak about
one hour after the start, and was subsequently on decline.
Effusion of lava began simultaneously with, or shortly
after, the explosive activity. By the second day of
eruption, the activity was already mainly restricted to a
single fissure on the lower south flank, trending southeast
from the top of Hekla. Slight activity in another fissure on
the south side of the volcano lasted until 20 January
(Gu6́mundsson et al. 1992).
The amplitude of the volcanic tremor rose rapidly and
stayed highest during the first hour. The maximum
reduced displacement (e.g. McNutt 1994a) calculated
from the records of the station SAU was about 8 cm2. The
effusion rate of lava (see below), a basaltic andesite, may
have reached a peak of 2,000 m3/s during the first day.
The average effusion rate during the first two days was
800 m3/s. Later on the rate declined rapidly to 10–20 m3/s,
then more slowly to 1 m3/s, and remained at 1–12 m3/s
until the end. The volcanic tremor diminished rapidly in
the evening of 10 March, and disappeared into the
background noise around 05:00 on 11 March. The
eruption most likely came to an end around noon on 11
March (Gu6́mundsson et al. 1992).
In the case of Hekla, the volcanic tremor appears to
more or less reflect the eruptive activity. It starts
simultaneously with the eruptions and dies away towards
the end of the eruptions, together with the volcanic
activity (Einarsson and Bjrnsson 1976; Grnvold et al.
1983; Gu6́mundsson et al. 1992). No tremor has been
observed from Hekla during non-eruptive times. This
behaviour is not self-evident for every volcano. Some
volcanoes show periods of volcanic tremor without
observable eruptive activity (Schick 1988). At Etna, for
example (e.g. Montalto et al. 1995), tremors are recorded
a long time before visible volcanic activity and probably
can be used as a tool for eruption prediction.
After the first hours of the Hekla eruption, the swarm
of numerous, small earthquakes died out and the volcanic
tremor was the main seismic expression of the eruption.
During later phases of the eruption, earthquakes at Hekla
were few (further details of the earthquake seismicity are
given by Soosalu and Einarsson 2002).
Tremor observations for the first hours
of the eruption
Representative samples of seismograms of SAU and ASM
during the first hours are presented in Fig. 2a, b. During
the first minutes of the assumed start of the eruption,
higher frequencies (i.e. earthquakes) are distinct on the
seismograms. Low-frequency volcanic tremor starts to be
visible first between earthquakes and then as a dominating
factor from ~17:05 on. The amplitude of the tremor
oscillates. Episodes of high-amplitude tremor, lasting
from some seconds to about 10 s, are separated by
moments of lower amplitude. This pattern is preserved
until midnight.
The tremor amplitude rises high rapidly after the onset
of the eruption. The maximum is reached only about
10 min after the start. From then on, the amplitude
remains high for about 1 h. Shortly before 18:00 it begins
to decline. From 18:00 on, the decline is quite rapid and
this tendency continues until 18:40–18:50. Subsequently,
the tremor declines much more slowly.
On the ASM seismograms the tremor is not as
outstanding as it is on SAU, although its general
appearance is similar. During the first minutes the
numerous earthquakes related to the onset of the eruption
dominate the appearance of the ASM seismograms. Later
on the low-frequency tremor is clearly visible and remains
in the seismograms until midnight. However, its amplitude diminishes quickly; there is a clear change already
after the first hour of the eruption.
565
Fig. 2 a Representative 2-minlong samples of SAU vertical
component records at the time
of the onset of the Hekla eruption, 17 January 1991. The
amplitude scale is the same for
every sample. No filtering is
done. b Representative samples
of ASM vertical component
records, times same as at SAU.
The amplitude scale is the same
for every ASM sample, and
twice the scale of the SAU
samples. No filtering is done.
The seismometer instrumentation is the same at both SAU
and ASM
Figures 3a–c and 4a–c show the temporal development
of three frequency bands of SAU and ASM data; both the
vertical and the two horizontal components are shown. As
the station SAU is located west of Hekla, its east
component was readily taken as the radial, and its north
component as the transverse component. The horizontal
components of the station ASM were rotated 20 counterclockwise to approximately obtain the radial and the
transverse components. The frequency bands 0.5–1.0,
1.0–2.0, and 2.0–4.0 Hz were considered relevant for
separating the fingerprints of the tremor (primarily the
0.5–1.0 Hz band) and the numerous earthquakes (the
higher frequencies). The insets show the behaviour in the
very beginning. At 17:02–17:03 GMT there is a sudden
increase in the 0.5–1.0 Hz tremor build-up, simultaneously with a decrease in the higher frequencies which
indicates decrease in earthquake activity. This suggests
that the fissure has stopped propagating and the magma
has a free path to the surface.
The tremor waves (low frequencies) attenuate more
with distance than the earthquake waves (higher frequencies). This is well visible both in the seismograms
(Fig. 2a, b) and in the attenuation of the different
frequency bands in Fig. 3a–c of SAU and Fig. 4a–c of the
more distant ASM.
The behaviour of the tremor during the first hours of
the eruption is quantitatively presented in a graph
expressing its intensity at SAU and ASM (Fig. 5a). The
vertical component data are used and band-pass filtered
between 0.5 and 5.0 Hz to eliminate the effect of the
microseism in the lower end and of the earthquakes in the
higher end. The intensity is calculated as averaged energy
over 60-s intervals. Its ln logarithm in function of time is
plotted on the graph. The general intensity pattern at both
stations is similar but overall smaller values of ASM are
566
Fig. 3a–c Tremor amplitude
time series with different frequency bands, 0.5–1.0 Hz (solid
line), 1.0–2.0 Hz (dashed line),
and 2.0–4.0 Hz (dotted line) for
the station SAU: a the vertical,
b the “radial” (E-W), and c the
“transverse” (N-S) component.
The values are one-minute amplitude averages. The insets
show the behaviour of the frequency bands around the very
onset of the eruption
due to greater distance. The continuous fluctuation of the
intensity throughout the time period is also common for
both. The earthquakes observed until midnight are in the
same plot for comparison.
At the onset of the eruption the intensity at SAU and
ASM jumps quickly, in matter of minutes, to high levels.
The intensity remains high during the first hour. At
18:10–18:20 it turns to a sharp decline which continues to
about 19:00. At that time there is a clear bend in both of
the diagrams, after which the intensity declines more
gently towards midnight. For comparison to the intensity
graphs, a more qualitative amplitude plot measured from
paper seismograms of the analog station LJ is drawn in
Fig. 5b. Tremor amplitudes and their decline at LJ follow
a similar pattern as the intensities at SAU and ASM.
Linde et al. (1993) analysed data from a strain station
network in the vicinity of Hekla during the eruption. They
observed at the nearest station, BUR, 15 km from Hekla
(see Fig. 1), remarkable contraction from the time before
16:40 on. This continued until 18:40 when a minimum
was reached and the strain turned to expansion. The more
distant stations, 37–45 km from Hekla, showed solely
expansion during the first two days of the eruption. They
interpreted this as two stages, of which the first one is
characterized by deflation of a deep reservoir (seen by the
distant stations) together with dyke formation (seen only
by the nearest station). The second stage, starting at
18:40, consists only of deflation of the deep source which
is observed by all the stations. The turning point of the
strain data at the nearest station is not reflected in an
obvious manner in the seismic intensity graphs (Fig. 5a),
567
Fig. 4a–c Tremor amplitude
time series with different frequency bands, 0.5–1.0 Hz (solid
line), 1.0–2.0 Hz (dashed line),
and 2.0–4.0 Hz (dotted line) for
the station ASM: a the vertical,
b the “radial”, and c the
“transverse” component (horizontals rotated 20 counterclockwise from original). The
insets show blowups of the
onset of the eruption
as the quick decline of intensity slows down not until
some 20 min later. However, the strain reversal coincides
with a local minimum of earthquakes.
Spectral analysis of SAU and ASM data
We used continuous records of the vertical components of
the stations SAU and ASM to study the frequency content
of the Hekla tremor during the first hours. The tremor has
a fluctuating nature, not only in the amplitude but also in
its spectrum, as the characteristic spectral peaks typically
pop up, remain a minute or a couple of minutes, disappear
or get small for some minutes, and reappear as major
peaks. We made continuous spectrograms, with the
resolution of 0.05 Hz, by making non-overlapping spectra
over 1-min-long samples of the records. These are shown
both in three-dimensional view (Figs. 6a and 7a) and in
contour plots (Figs. 6b and 7b). We also made a
schematic spectrogram for the station SAU by stacking
the spectra of five successive minutes (Fig. 8).
Both the stations observed that the tremor was within a
well-defined spectral band at 0.5–1.5 Hz. This is in the
lower end of the range which has been observed for
volcanic tremor in general, mainly 1–9 Hz at active
volcanoes around the world (McNutt 1994b). The spectra
settled to this band very quickly, only a few minutes after
the onset of the eruption. The spectra remained like this
all the time during the first 7 h of the eruption. The tremor
was sharply peaked—typically there were one dominant
peak and a couple of other, outstanding peaks. The
amplitudes of the peaks decreased with time but the
spectral range remained the same.
568
Fig. 5 a Tremor intensity observations of SAU and ASM, 17
January 1991. For the graph the
seismic data of the vertical
components are band-pass filtered between 0.5 and 5.0 Hz.
The intensity is calculated as
averaged energy over 60-s intervals, and its ln logarithm is
plotted. The earthquakes related
to the onset of the eruption are
presented below, their magnitude scale is on the right side.
The eruption started at about
17:00. The moment of the
turning point at the strain station BUR is marked with an
arrow. b The amplitudes of the
analog station LJ are shown for
comparison. In this plot the
shaded area shows the representative peak-to-peak amplitudes, and the solid line the
maxima during 10-min-long
periods
At the nearer station, SAU, the spectrum remained at
its characteristic band from about 17:05 on until the end
of the dataset (~23:30). Practically all the maximum
peaks were at the band of 0.7–0.9 Hz, however, during the
first 2 h in the lower half, and later in the upper half of
this band (Figs. 6a, b and 8). Most of the notable peaks
existed at the band of ~0.6–1.2 Hz.
During the first hour, until 18:10, the spectral peaks of
SAU had their highest amplitudes. All the maximum
peaks were at 0.6–0.8 Hz, and a peak at 0.75 Hz remained
highest most of the time. Prominent peaks existed also at
0.7, 0.8, 0.85 and 0.9 Hz. At 18:00 the dominant peak
switched to 0.7 Hz where it continuously stayed until
18:35. Then, suddenly the maximum peak switched to
0.9 Hz for some 10 min. This happens also to be the time
when strain at the strain station BUR went from
contraction to expansion (Linde et al. 1993). Later the
location of the maximum peaks switched back to 0.7 Hz
and stayed there until 19:00 but after that the location of
the highest peak varied in the band of 0.7–0.9 Hz. Around
19:00 all the maximum peaks “disappeared” for a few
minutes, as the peaks had considerably smaller amplitudes than during the minutes before and after (Fig. 8).
This was the time of a clear kink in the intensity graph of
SAU (Fig. 5a).
After 19:20 a persistent dominant peak appeared at
about 0.85 Hz in the SAU spectrum (Fig. 6a, b). It
disappeared for some minutes but then reappeared again
at the same frequency. However, until about 20:40, the
0.7 Hz peak occasionally dominated (often when the
0.85-Hz peak was momentarily absent). Thus, the major
spectral peaks still were within the same main band, 0.7–
0.9 Hz, as earlier but the dominance had switched to 0.8–
0.9 Hz. From about 20:40 the persistent major peak of the
SAU spectrum was the one at about 0.85 Hz. It fluctuated
slightly a few hundredths of hertz and sometimes the
maximum skipped to 0.8 or 0.9 Hz. However, the other
spectral characteristics, i.e. typical frequency band, and
location and existence of typical peaks, continued until
the end of the dataset. In the SAU spectrum the high
semicontinuous peaks at 0.7 and 0.85 Hz are most
apparent. Some other peaks are also prominent at 1.0,
1.25, and 1.45 Hz, although they are never as dominant as
the ones at lower frequencies.
The overall structure of the spectrum of the Hekla
tremor at the station ASM, 15 km further away, is similar
to that of SAU but in the details the spectra differ. The
569
Fig. 6a, b A spectrogram of the
vertical component data at the
station SAU for the first hours
of the eruption: a three-dimensional view, and b plane view
with contour lines (the amplitude scale is arbitrary). The
spectrum is a fast Fourier
transform power spectrum. The
data consist of detrended and
demeaned 60-s-long non-overlapping time windows with a
0.5-s cosine taper. No filtering
is done. The abscissa is the
frequency and it is cut at 4 Hz
because peaks above it are of
negligible size. The ordinate is
the time, from 16:30 on 17
January until midnight
tremor quite effectively attenuates with distance, as the
Hekla earthquakes appear more clearly amid the tremor at
ASM than at SAU. During the first minutes after 17:00,
the numerous earthquakes dominated the ASM spectrum.
Later the tremor took over, although the largest earthquakes continued to show their fingerprint in the ASM
spectrum. Throughout the dataset, the location of maximum peaks within the band varied somewhat, although
the same peaks existed most of the time. The most
persistent peaks were 0.75 Hz at 17:40–20:00, 0.85 Hz at
18:45–24:00, and 1.2 Hz at 20:30–23:45. No tremor peak
was as persistently dominant as the one at 0.85 Hz at
SAU.
In the beginning, at 17:05–18:00 when the tremor was
at its maximum, the highest spectral peak at ASM was at
about 0.9 Hz. A more persistent peak, but not as high,
existed at 0.7 Hz from 17:05. It occasionally popped up
until 18:20, after which it was not prominent. At about
17:20 a similarly persistent peak at 0.75 Hz entered but
this one survived with varying size until the end of the
dataset. Another clear peak at 1.1 Hz existed in the
spectrum during the first hour of the eruption.
At the time when the tremor intensity started to
diminish, from about 18:00 (Fig. 5a), the band of maxima
at ASM widened to 0.75–1.3 Hz. During the quick
decrease of the intensity, ca. 18:00–19:00, the maximum
570
Fig. 7a, b A spectrogram of the
vertical component data at the
station ASM for the first hours
of the eruption: a) three-dimensional view, and b plane
view with contour lines. The
data processing is the same as in
Fig. 6. Arbitrary amplitude
units are the same as for SAU.
Note that the amplitude values
are far smaller compared to the
SAU observations
peaks were located at 0.75, 1.0, 1.1, and 1.3 Hz. At 18:40
the strain station BUR (Linde et al. 1993) had its turning
point, coinciding with a clear decline of amplitudes of the
spectral peaks of ASM. From 18:40 to 19:10, the main
maximum was at 0.75 Hz, and this peak kept on existing
as a prominent one until midnight. After about 19:00, the
peak at 0.85 Hz appeared in the spectrum of ASM as an
occasional maximum. At about 19:00, a kink is visible
also in the ASM tremor intensity graph (Fig. 5a). This is
not reflected in the spectrum at ASM as dramatically as at
SAU. However, at this time the 0.85-Hz peak started to be
a major one. After about 19:00 until midnight, distinct
peaks were at 0.75, 0.85, 1.0, 1.1, 1.2, and 1.4 Hz.
The spectra of SAU and ASM have both common and
different elements—the general appearances are similar
but details differ. They both reflect the major turning
points of the first hours of the eruption—the turn of the
tremor intensity to rapid decrease, the strain change at the
local strain station, and change to gentle decrease of the
tremor intensity—but differently. Also, the characteristic
peaks are similar but appear in different patterns. At SAU
they are 0.7, 0.8, 0.85, 0.9, 1.0, 1.25, and 1.45 Hz; at ASM
the values are 0.7, 0.75, 0.85, 1.0, 1.1, 1.2, 1.3, and
1.4 Hz. Some peaks coincide exactly, e.g. 0.85 Hz, but the
higher ones at 1.2 or 1.25 Hz and at 1.4 or 1.45 Hz do not.
571
Fig. 8 Stacked spectra of SAU
observations. From 17:00 on,
each successive five spectra of
60-s-long records are summed
up and prominent peaks presented in a semiquantitative
manner. The largest dot represents the dominant (on some
occasions two dominant)
peak(s), the next largest dots
prominent peaks, the third largest dots quite prominent ones,
and the smallest dots peaks
which are not very prominent
but still distinct. The peaks of
each five-spectra stack are
compared to each other, not to
peaks at other times. The only
exception is the highest peaks at
19:00–19:05 which are marked
just with symbols as “quite
prominent” because they are
considerably smaller than peaks
immediately before and after.
The shaded bands show the
main area of the spectrum: the
darkly shaded is the band of
outstanding peaks, the middle
shading the band of prominent
peaks (including the former),
and the lightly shaded band is
the area of “visible” spectrum
when scaled with the maximum
peaks. The intensity graph of
SAU and earthquakes of Fig. 5a
are shown for comparison.
Physical phenomena which can
be linked to the observations are
suggested in the figure
The different distance from Hekla certainly affects the
appearance of the spectra, as is seen from the different
attenuation of the tremor and the earthquakes. Different
geological conditions below the stations or along the path
of seismic signals may magnify the peaks in different
ways. The seismic rays to ASM have to travel a longer
path via the heavily faulted South Iceland seismic zone
than those going to SAU. Thus, the 0.85-Hz peak which is
distinct at both stations after the first 2 h of the eruption is
very dominant at SAU but exists much more modestly at
ASM. On the other hand, the higher frequency peaks (at
1.0–1.45 Hz) are more prominent compared to the lower
frequency peaks (at 0.7–0.9 Hz) at the more distant ASM
that at the nearer SAU.
Two analog stations approximately 80 km W–WNW
of Hekla also recorded the tremor related to the onset of
the eruption. The tremor is clearly visible until about
18:30 on both, then it quickly decreases until 19:00. From
then on it disappears into the noise. The general pattern of
the tremor decline is thus similar at all the observing
stations.
The beginning phase of the eruption, first days
This period is best documented by the paper seismograms
of the analog station LJ, 35 km ENE from Hekla (Fig. 1).
It was the nearest analog station observing the start of the
Hekla eruption. The amplitude of the tremor recorded by
LJ was measured at 10-min intervals from 17 January on,
until it faded out below the background level a few days
later. The initial onset of the tremor is difficult to
572
Fig. 9 Eight-second-long samples of vertical component
seismograms at HAU, with no
filtering. The amplitude scale
(tick marks at €0.15
105 m/s)
in all the plots is the same,
except that the 17 January
sample is plotted at reduced
scale (10 times larger than the
others). Date and time of the
samples are given. Some seismograms (e.g. 27 January) have
minor high-frequency noise
determine from these records because the lines are mixed
with each other. Reliable measurements can be made
from 17:20 on. The tremor amplitude between 17 January
at 17:00 and 18 January at 02:00 is presented in Fig. 5b.
The first hours before midnight show a similar pattern as
the intensity values of the stations SAU and ASM
(Fig. 5a).
During the period 02:20–08:10 on 18 January, the
magnification of LJ had been turned down and it is not
possible to measure the tremor amplitude then. After
08:20, the tremor amplitude remained at the same level as
in the first hours of 18 January. However, the slight
declining trend of the tremor amplitude continued, but an
increase was visible during the period 10:20–18:50 on 19
January. During the next days, the tremor kept on
declining gradually and, from 21–22 January on, it was
difficult to distinguish between the tremor and the
background noise. The records of the station LJ are in
harmony with the observation of Gu6́mundsson et al.
(1992) that the eruption was most vigorous during the first
11 h.
The tremor and its variation during
the whole eruption
The digital station HAU is the nearest SIL station to
Hekla, and is located 15 km west of the volcano. It
recorded only one short sample of data on 17 January but,
from 19 January on, there is a tremor dataset of
intermittent HAU observations for almost every day of
the eruption. The data collected by HAU are the principal
material of the study of tremor during the eruption as a
whole. In most cases the HAU files which are available
were saved because of events, i.e. the files contain
earthquake data and some preceding seconds. Occasionally, 1-min-long files containing solely tremor were also
saved. Eight-second-long tremor samples were selected
for further studies, in order to maximize the amount of
useful data.
Generally there are several observations per day but
some days lack data. A representative set of vertical
component seismograms of HAU is shown in Fig. 9. The
tremor was very vigorous on the seismogram of 17
January, and for convenience it is drawn here at a scale
10 times smaller than the other samples. However, the
appearance of the waveform is remarkably similar on that
day compared to the following days. During the first days,
the tremor was clearly visible as a major element of the
seismograms. Its amplitudes fluctuated day by day, and
also within one and the same day. The first distinct
decline occurred on 22 January. However, the general
level of the tremor remained quite high also in the
following days.
The volcanic tremor was a major element of the
seismograms until about 27 January, after which its
amplitude clearly declined. However, around noon on 31
January, there was a distinct increase in the tremor. By the
following day this had already subsided. From the
beginning of February on, the low-frequency oceanic
microseism started to dominate the seismograms, and the
highs and lows of the tremor were occasionally detectable. In the beginning of March the tremor was visible
on the seismograms only now and then. The tremor
disappeared below the detection threshold of HAU, i.e.
about 0.7–1.4
107 m/s, a few days before the end of the
visible eruption.
The intensity of the tremor during the whole eruption
was determined, in the same way as for the first evening
in Fig. 5a, from all the 8-s-long samples of the tremor
573
Fig. 10 a Effusion rate throughout the eruption (from Gu6́mundsson et al. 1992). b Semicontinuous tremor intensity observations of
HAU throughout the eruption, from the vertical component data.
The sample length was 8 s but the method for calculating intensity
was the same as used for SAU and ASM in Fig. 5a. The thick arrow
on 11 March shows the end of the eruption. c HE amplitude
observations in the last days (starting on 24 February). In the HE
plot the shaded area shows representative peak-to-peak amplitudes,
and the solid line the maxima. There are observation points
approximately once a day. Thin arrows pointing towards their
moment of occurrence show periods of increased tremor level at
analog stations LJ (arrows in January–early February) and HE (late
February–March). The length of the arrows tells the length of the
periods of increased activity, so that the shortest arrows mark
20 min of increased activity, the longest arrow 110 min
which were available. The data are band-pass filtered in
0.5–5.0 Hz, and the averaged energy over the 8 s is
calculated. This gives a semicontinuous image of the
intensity; the corresponding ln logarithm values are
presented in Fig. 10b. The observations of 17 January
(at ~20:28) give values many orders of magnitude higher
than at later times. This is in agreement with the
observations of most vigorous tremor and effusion rate
during the first hours of the eruption. From 19 January on,
the intensity shows rather large variation. There is a
general diminishing trend throughout the eruption. On the
other hand, comparatively high short-term fluctuations in
the tremor amplitude and intensity seem to be characteristic for the whole eruption. The intensity values can
fluctuate considerably even within one minute (Fig. 10b).
Sometimes there were periods of clearly increased
intensities, i.e. on 25–26 February.
Gu6́mundsson et al. (1992) made observations of the
effusion rate during the Hekla eruption and they present
the results in a schematic graph, which is here combined
with our tremor intensity data in Fig. 10a. They remark
that during the eruption its vigour and the tremor
amplitudes correlated. However, the qualitative nature
of their graph must be kept in mind when comparing it to
the tremor intensity values. The effusion rate graph was
constructed by using visual observations, and there are
inaccuracies, e.g. due to poor weather conditions or
occasional gaps of some days between successive observations.
In general, features of the intensity observations at
HAU and the effusion rate graph show similar behaviour.
Both have high values in the very beginning of the
eruption, and both calm down to much lower level quite
rapidly. Later the more slowly declining trend is visible at
both until the end of the eruption. However, in details the
graphs have some differences. Partly this can be caused
by inaccuracy of the effusion rate diagram but differences
can also be real. For comparison, Cristofolini et al.
(1987), for example, observed volcanic tremor at the
Sicilian volcano Etna in June 1980–March 1981 when
several eruptions took place. They noted that the highest
tremor levels did not always coincide with eruptive
phenomena.
574
Although the volcanic tremor on records of LJ had
faded out after a couple of days from the onset of the
Hekla eruption, LJ managed to detect some later periods
of increased tremor activity. These periods occurred in
late January and early February and lasted 20 to 110 min.
They are marked with arrows in Fig. 10b. At the end of
the eruption, from 24 February on, the analog station HE
which is located on Hekla itself is also available. HE has
observed similar periods of increased tremor as LJ did in
the earlier days. These periods are marked with arrows in
Fig. 10b, c as well, and lasted 20 to 60 min. It is
somewhat difficult to connect these observations of
increased tremor at the analog stations to the intensity
values of HAU. They are rather short, and there are only
two observation points of HAU (on 2 February and 9
March) which coincide with the analog records (these two
show rather high local values). It is possible that the
momentary increase of tremor observed by the two analog
stations is typical for this eruption. During the times when
the tremor was still at moderate level, and LJ could detect
a considerable increase, LJ often picked such periods.
Also when HE was in use, this seemed to be a frequent
phenomenon. During the days in between, there was no
suitable station available to detect them.
The very end of the eruption is probably best observed
by HE (Fig. 10c). The tremor faded out earlier from the
records of HAU 15 km away than from the records of HE
on the flank of the volcano. When HE observations start,
on 24 February, the volcanic tremor was well visible on
its records. During the first days of HE recording, there
was a period of general increase in tremor in the
observations of HAU. The amplitude values of HE and
the intensity values of HAU decline similarly towards 3
March. After that the amplitude remains small and even,
however, clearly detectable on HE. The values of HAU
still show distinct tremor on 5–7 March, but later on it is
hard to say if the tremor can be observed. On HE,
although the amplitude values are small, the tremor can
still be distinguished until 10 March. The tremor finally
disappeared under the detection threshold of HE during
the early hours of 11 March. The end of the tremor was
followed by a swarm of small earthquakes in the morning.
Gu6́mundsson et al. (1992) put 11 March, at about noon,
as the end time of the eruption.
Due to the scarcity of the data we could not make a
spectral study of the tremor during the later days of the
eruption. Gu6́mundsson et al. (1992, p. 242) describe
results of a small seismic array which was installed near
the active crater and operated one day. They detected
narrow-band random noise with peak amplitude at about
2.5 Hz, higher than our observations for the first day.
They found the tremor consisting of Rayleigh and Love
waves with very low velocities.
Particle motion at the onset of the eruption
The source of volcanic tremor is often inferred to be
shallow, and the tremor thus consists mainly of surface
Fig. 11a, b Particle motion of a 5.0-s-long sample from station
HAU at 20:28:05 on 17 January 1991 (see Fig. 9). The data were
band-pass filtered at frequencies 0.5–1.0 Hz, the trend and the mean
were removed, and a 0.5-s-long cosine taper was used. a The
motion in the radial-vertical plane. b The motion in the radialtransverse plane. The units are arbitrary, and arrows mark the
direction of the movement
waves (e.g. McNutt 1986; Gordeev et al. 1990; Gordeev
1992; Goldstein and Chouet 1994; Ripepe et al. 1996).
We made particle motion plots for the station HAU. As
Hekla, the source of the tremor, is located roughly east of
the station, the horizontal north–south and east–west
components can be taken approximately as transverse and
radial components, respectively. In Fig. 11a, b there is an
example of particle motion diagrams of a 5-s-long sample
of HAU records at 20:28 on 17 January, in the frequency
band 0.5–1.0 Hz. Both of the graphs give somewhat
diffuse pictures of the wave content of the Hekla tremor.
However, in the radial-vertical plot (Fig. 11a), the
retrograde elliptical movement of the Rayleigh waves is
obvious. In the radial-transverse plot (Fig. 11b), the
particle motion has a clear component of movement in E–
W, the direction towards Hekla, this being another
indication of possible Rayleigh waves. The particle
motion was quite variable in our plots, e.g. a sample at
HAU 15 s earlier did not show these motions very clearly.
Apparently mixed modes of different wave types are
present in the Hekla tremor.
575
The frequency band plots of all the components of
SAU and ASM (Figs. 3a–c and 4a–c) also point to the
existence of surface waves. At the frequency bands of
0.5–1.0 and 1.0–2.0 Hz, the attenuation of the vertical
component is much bigger than that of the horizontals
(compare amplitudes at ASM in Fig. 4a–c). The transverse has highest amplitudes of the three components at
both SAU and ASM in the two lower frequency bands,
especially in the frequency band of 0.5–1.0 Hz. This is the
component where horizontal shear waves and Love waves
are best visible, and likely a sign of their existence. This
could indicate that Love waves become more important
with increasing distance. For comparison, Ereditato and
Luongo (1994) have made a study on volcanic tremor at
Etna, and they likewise found a large amount of Love
waves.
Conclusions and discussion
A number of possible causes for volcanic tremor have
been proposed in the literature. There are models which
explain the tremor as the result of resonant effects
produced by the geometry of volcanic conduits. Turbulent
motion in the vapour–gas–magma mixture makes the
volcanic pipes oscillate (e.g. Seidl et al. 1981; Ferrick et
al. 1982). In such a case, the frequency content of the
tremor may vary with the length of the conduit. Other
models suggest that volcanic tremor is produced by
vibrations of tensile, fluid-filled, jerkily or suddenly
opening cracks (Aki et al. 1977; Chouet 1981). In such
models the excess pressure in the fluid generates the
trembling. Active participation of the fluid in the form of
degassing was added to this modelling by Chouet (1985).
In another study, Chouet (1992) states that the tremor is
the response of the tremor-generating system to sustained
bubble oscillations in the fluid.
Volcanic tremor is often observed to have a peaked
spectrum with typically one dominant and a few subdominant peaks (e.g. Aki et al. 1977; Chouet 1981; Seidl
et al. 1981; Ferrick et al. 1982; Chouet 1985, 1988, 1992).
According to Aki et al. (1977), the peaks can be explained
as a source effect, such as free oscillations of a magma
chamber, or path effects, such as reverberations from the
stratigraphic layers. Temporal change of peak frequency
during the course of volcanic activity would favour source
effect explanations. According to Chouet (1985, 1988),
the location of the major spectral peak is dependent on the
fluid viscosity and the stiffness of magma-filled cracks.
Gordeev et al. (1990) studied volcanic tremor with an
array of seismographs at the Klyuchevskoy volcano in
Kamchatka. They concluded that the general envelope of
the tremor frequency band represents its source, and
individual spectral peaks rather reflect individual properties of the medium at the observing stations. Thus, the
medium near the stations would act as a stratigraphic
filter, and the observed peaks can be quite different at
different locations.
Volcanic tremor often begins prior to the actual surface
outbreak of an eruption and may extend beyond the
duration of surface activity (e.g. Chouet 1981; Montalto et
al. 1995). This was not the case at Hekla where the tremor
started at the same time as the eruption, and also stopped
simultaneously with the eruptive activity. Thus, the
tremor observations cannot be used for predicting the
Hekla eruptions. The tremor-producing process at Hekla
could apparently not start before the connection of the
magma conduit with the atmosphere was reached. This
supports the explanation of degassing of magma being a
cause of the tremor.
During the first hours of the Hekla eruption, the fissure
system was changing dramatically and several fissures
were active. The effusive activity diminished significantly
during the first night and was getting more localized
(Gu6́mundsson et al. 1992; Gu6́mundsson and Saemundsson 1992). Although the eruptive activity and
tremor intensity were quite variable, the main frequency
band of the tremor, both at SAU and ASM, was
remarkably stable at about 0.5–1.5 Hz during the first
7 h. However, the individual outstanding peaks within the
band were amplified differently at different stations, and
the relative amplitude of the peaks changed with time.
The idea of Gordeev et al. (1990) of a general spectral
envelope of the volcanic tremor and separate peaks being
local effects near the stations can be valid to some extent.
Still, there are patterns in the locations of the spectral
peaks which can be associated with known phenomena in
the course of the eruption (see Fig. 8). Thus, we believe
that although the station SAU is quite distant from Hekla,
we can see features in the tremor records which are source
effects. Admittedly, path effects and source effects cannot
be distinguished very well with our dataset. For instance,
many patterns which are clear in the spectra of SAU get
blurred in the spectra of the more distant ASM with a
different path.
The dominant peaks at the lower frequency end
(<1 Hz) of the characteristic spectral band of the tremor
suffer more from attenuation than those in the higher
frequency end (>1 Hz). This is surprising, as it is
generally assumed that high frequencies attenuate faster
than the low ones. Ukawa (1993) has made similar
observations as ours at the Ito-oki submarine volcano in
Japan. During the 1989 eruption of Ito-oki, volcanic
tremor was recorded, with two kinds of predominant
spectral peaks, around 1 Hz and between 2 and 7 Hz.
Low-frequency waves were dominant at distances less
than 50 km, and the higher-frequency waves at more
distant stations. Ukawa (1993) states that the former are
signals of surface waves trapped and magnified in the
sedimentary layers of the sea bottom, and the latter
express compressional body waves travelling deeper in
bedrock.
If the frequency content of the tremor is dependent on
the size of the magma channel, as in the models of Seidl
et al. (1981) and Ferrick et al. (1982), a shift of maximum
spectral peak should be visible in the beginning phase of
the Hekla eruption. Thus, the peak should be at a lower
576
frequency when there is a larger fissure open, and then
change to higher frequency when the magma channel gets
smaller.
The general observation that tremor is sharply peaked,
having one dominant and several subdominant peaks, is
valid for Hekla. The spectral peaks of SAU provide an
opportunity to relate the course of the eruption to the
tremor (Fig. 8). During the first couple of minutes of the
eruption, the numerous earthquakes dominate the spectrum with higher frequencies but, soon, the main peaks
jump to the characteristic band of the tremor, 0.7–0.9 Hz.
The main peaks remained there the following hours, first
in the lower end, 0.7–0.8 Hz, and later in the higher end,
0.8–0.9 Hz. During the first tens of minutes of the
eruption when the fissure was opening, the main peak was
at 0.75 Hz. After 17:50, the ML 2.5 earthquake occurred
which could be the sign that the fissure had reached its
maximum. At this time the major spectral peak jumped to
lower frequency, 0.65 Hz. During the first half an hour
after 18:00, the main peak was at 0.7 Hz, and this can be
interpreted to be the time when the fissure was at its
biggest. After that the intensity of the tremor started its
rapid decrease. After 18:30 the major peak at 0.9 Hz
appeared for the first time. Soon after this, the strain
station BUR observed a change which showed that the
dyke had stopped growing. The fissure started to close
and to be divided into smaller segments. The main
spectral peak remained at 0.7 Hz. The spectrum changed
drastically around 19:00, at the time of a kink in the
intensity diagram. For some minutes all the spectral peaks
became considerably smaller than immediately before and
after. The peaks soon recovered but after this they had a
more diffuse pattern and the main peaks at 0.8–0.9 Hz
became common. Occasional dominance of the 0.7-Hz
peak more or less stopped around 20:40, and later the
dominant peaks were at 0.8–0.9 Hz. This could indicate
that the fissure remained quite stable as from 20:40. In
Fig. 6a, b the outstanding peak is at 0.85 Hz. However, in
the stacked plot in Fig. 8, the main peak fluctuates at 0.8–
0.9 Hz. This is caused by the fact that the 0.85-Hz peak
seldom was continuous for longer than a couple of
minutes.
Tremor observations of Gu6́mundsson et al. (1992)
compared to ours fit, at least qualitatively, to the idea that
large eruptive channels produce lower peak frequencies
and small channels higher frequencies. Their study was
made in a later stage of the eruption when the activity was
restricted to one crater, and it showed a higher peak
frequency (2.5 Hz) than ours.
Chouet (1981) states that a shift in the dominant
tremor frequency does not have to reflect a growing crack
but may result from a change in source location, i.e. a
change in the path of magma during the course of the
eruption. Also, according to Chouet (1985), lowering the
fluid viscosity sharpens and amplifies the dominant and
subdominant peaks, and also shifts the frequency to a
higher value. Further, Chouet (1988) mentions that the
crack stiffness affects the tremor spectrum. Larger
stiffness shifts the peaks to lower frequencies and
decreases their bandwidth.
Rapid changes were characteristic of this Hekla
eruption. The earthquakes preceding the onset of the
eruption were observed only about half an hour earlier.
After the eruption had reached the surface and the tremor
started, its spectrum settled in some minutes to its
characteristic band at about 0.5–1.5 Hz. On the other
hand, the fluctuating nature of the Hekla tremor, both in
the short and long term, appears to be an intrinsic feature.
This is seen in the seismograms, intensity graphs and
spectra throughout the eruption. As stated in Seidl et al.
(1981) for the tremors at Etna, the amplitude frequently
shows sudden transients where the average amplitude
level increases for a few seconds. “Wave packages”
lasting some seconds and which are seen in the seismograms of Fig. 2a, b could be similar to these. Seidl et al.
(1981) explain this to be caused by transient hydraulic
pressure pulses in the magma flow.
The volcanic tremor is sometimes described to be
“harmonic” (e.g. Chouet 1985; McNutt 1986; Chouet
1988; Schlindwein et al. 1995) when evenly spaced
spectral peaks represent eigenfrequencies, similar to
organ pipes. This does not seem to be valid for Hekla.
The earthquakes of the Hekla area attenuated less with
increasing distance than the tremor, as is seen in the
observations of SAU and ASM. The difference in
attenuation suggests that the earthquakes originated
deeper than the tremor. The tremor waves have travelled
in loose and faulted surface layers, and the earthquake
waves in deeper solid layers with lower attenuation.
Effusion rate and tremor intensity during the eruption
correlated in a general way but there were differences in
details. Although the effusion rate graph in Fig. 10a has
an approximate nature, some of the dissimilarities can be
real, e.g. a bulge in the effusion rate curve on 11–23
February which is not reflected in the intensity in
Fig. 10b. However, rather than comparing these figures
to each other and trying to see correlation between them,
one should think of them as representing two, somewhat
different sides of the magma dynamics. Ferrick et al.
(1982) state that only turbulent movement of the magma
flow is reflected by the tremor, and steady (or slowly
changing) fluid flow is aseismic. Therefore, it is not
possible to quantify the total volume of magma flow
using seismic data generated during episodes of volcanic
tremor. Studies of Cristofolini et al. (1987) indicate that at
Mt. Etna magma rises mainly aseismically through the
readily open main conduits, which agrees with the
generally poor correlation of earthquakes or tremor with
eruptive episodes.
After its short-lived, initial plinian phase, the Hekla
eruption was not very violent; further the earthquake
seismicity was modest after the very beginning. After the
first few days of the eruption, lava was observed
streaming in only one or a few channels. On occasions
the activity increased, and sometimes a thick steam
column and considerable explosive activity were observed. Schick (1988, 1992) states that strong tremor is
577
not necessarily accompanied by strong lava emission, but
strong degassing of volcanoes does coincide with strong
tremor. In the light of our observations on this Hekla
eruption, it is also likely that the Hekla tremor rather
reflects the vigour of the eruption than the amount of
produced lava.
Acknowledgements H. Soosalu was supported by Finnish Cultural
Foundation and the Vilho, Yrj and Kalle Visl Foundation of
the Finnish Academy of Science and Letters. The Icelandic
Meteorological Office provided the digital SIL data. The National
Power Company of Iceland funds the analog seismograph network.
Comments of two anonymous reviewers improved the manuscript.
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